Typically, laser apparatuses generate UV beams by directing a source beam to a non-linear crystal. Within the crystal, the source beam is frequency converted to a higher frequency, producing a shorter wavelength beam. For example, a 532 nm beam can be directed to a non-linear crystal to generate a 266 nm beam (the second harmonic wavelength of the 532 nm beam). For example, a 1064 nm beam and a 532 nm beam can generate a 355 nm beam (the third harmonic wavelength of the 1064 nm beam). Sometimes multiple crystals are necessary to convert a source beam to a desired UV beam wavelength.
Many processes require very stable UV beam parameters. Semiconductor wafer inspection processes, for example, tolerate less than 5 percent beam parameter drift over time. With UV processing, the beam parameter can drift as the non-linear crystal degrades. UV beam parameter drift manifests as unstable beam quality (M2), unstable axial beam waist location (z0), and unstable beam waist diameter (2ω0) (among others).
When source beams are frequency converted using a non-linear crystal, UV beam parameters can degrade due to bulk or surface degradation of the crystal. Bulk degradation can result from photo-assisted modifications of the crystal along the beam path inside the crystal and from related “compaction” of optical material. Bulk degradation causes increased thermal dephasing, and wavefront distortion, related to increasing absorption of the source and UV beams. Surface degradation can result from photo-assisted deposition and decomposition of contaminants of the crystal environment or by gradual destruction of the crystal surface (leading to unwanted wave front distortions or diffraction effects).
Existing solutions to address UV beam degradation include shifting the source beam to a new spot on the crystal when beam parameters hit specification limits (see, e.g., U.S. Pat. No. 8,976,343). This approach extends crystal life, but disadvantageously leads to step-wise changes in beam parameters after spot shifting, which can impact laser tool performance. This approach does not address unstable beam parameters.
Existing solutions also include continuous shifting of the non-linear crystal relative to the incident beam (see, e.g., U.S. Pat. No. 8,482,846). This approach can further prolong crystal lifetime but does nothing to address unstable beam parameters.
Although existing solutions prolong crystal lifetime by reducing degradation rates of the crystal, those solutions do nothing to stabilize beam parameters.